N-myc is a nuclear oncogene which is generally overexpressed in neuroblastoma. Although N-myc protein is DNA binding and is structurally similar to c-myc, its function in both normal and neoplastic cells is unknown. We are making use of the "antisense concept" to study the role of N-myc in neuroblastoma and neuroepithelioma cell lines which overexpressed this oncogene. This approach is two-fold: 1) addition to cell cultures of small single-sided oligonucleotides complementary to a portion of N-myc MRNA; and 2) transfection of cells with expression vectors containing a portion of the N-myc gene in antisense orientation. Using the former approach, we have been able to show that ohgonucletoide 15-mers complementary to an area containing the initiation site of N-myc MRNA are capable of concentrating in cells and inhibiting production of N-myc protein. In addition, these oligos inhibit cellular DNA synthesis and production of a nuclear protein termed Ki67. This protein may be a cofactor for DNA polymerase alpha and is required for DNA synthesis in isolated nuclei. Continued administration of antisense oligomer to neuroblastoma cells leads to reduced cell growth. 'Ibis growth inhibition is not accompanied by a reduction in c-myc protein and may be secondary to loss of the typical mixed morphologies normally seen in neuroectodermal cultures. In order to confirm these findings, we utilized the second approach, vector transfection, by constructing an antisense N-myc containing episomal vector which replicates extra-chromosomally at high copy number. Results obtained with this vector support the findings made with antisense oligos - namely, that N-myc suppression leads to a reduced growth rate of the culture; not due to a direct affect on growth, but to an alteration in the morphologic heterogeneity normally observed in neuroectodermal cultures in vitro. These results have implications concerning the mode of generation and maintenance of the mixed morphology cultures and the involvement of N-myc in this process.